var canvas = document.getElementById('canvas'),
	ctx = canvas.getContext('2d'),
	w = canvas.width = window.innerWidth,
	h = canvas.height = window.innerHeight,
	hue = 217,
	stars = [],
	count = 0,
	maxStars = 1300; //星星数量 
var canvas2 = document.createElement('canvas'),
	ctx2 = canvas2.getContext('2d');
canvas2.width = 100;
canvas2.height = 100;
var half = canvas2.width / 2,
	gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
gradient2.addColorStop(0.025, '#CCC');
gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
gradient2.addColorStop(1, 'transparent');
ctx2.fillStyle = gradient2;
ctx2.beginPath();
ctx2.arc(half, half, half, 0, Math.PI * 2);
ctx2.fill(); // End cache 
function random(min, max) {
	if(arguments.length < 2) {
		max = min;
		min = 0;
	}
	if(min > max) {
		var hold = max;
		max = min;
		min = hold;
	}
	return Math.floor(Math.random() * (max - min + 1)) + min;
}
 
function maxOrbit(x, y) {
	var max = Math.max(x, y),
		diameter = Math.round(Math.sqrt(max * max + max * max));
	return diameter / 2; //星星移动范围，值越大范围越小， 
}
var Star = function() {
	this.orbitRadius = random(maxOrbit(w, h));
	this.radius = random(60, this.orbitRadius) / 8; //星星大小    
	this.orbitX = w / 2;
	this.orbitY = h / 2;
	this.timePassed = random(0, maxStars);
	this.speed = random(this.orbitRadius) / 50000; //星星移动速度    
	this.alpha = random(2, 10) / 10;
	count++;
	stars[count] = this;
}
Star.prototype.draw = function() {
	var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
		y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
		twinkle = random(10);
	if(twinkle === 1 && this.alpha > 0) {
		this.alpha -= 0.05;
	} else if(twinkle === 2 && this.alpha < 1) {
		this.alpha += 0.05;
	}
	ctx.globalAlpha = this.alpha;
	ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
	this.timePassed += this.speed;
}
for(var i = 0; i < maxStars; i++) {
	new Star();
}
 
function animation() {
	ctx.globalCompositeOperation = 'source-over';
	ctx.globalAlpha = 0.5; //尾巴    
	ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 2)';
	ctx.fillRect(0, 0, w, h) ctx.globalCompositeOperation = 'lighter';
	for(var i = 1, l = stars.length; i < l; i++) {
		stars[i].draw();
		canvas2.style.cssText = "display:none";
	};
	window.requestAnimationFrame(animation);
}
animation();